1. Field of Invention
This invention relates to the field of switching-type battery chargers and specifically to a high power factor battery charger which is suitable for use in charging systems wherein a high output power and relatively small size is necessary.
2. Description of the Prior Art
Switching-type power supplies are noted for their high-efficiency, light weight, and probable long term cost advantages as copper and steel rise in price. Switching-type power supplies are especially useful in applications where heat dissipation and size are important.
Typically, a switching power supply achieves voltage regulation through the use of a solid state switch, such as a transistor, which is gated on or off according to the power requirement of the load. This technique known as duty cycle regulation is quite efficient as power is delivered to the load in proportion to power requirement of the load. The power delivered to the load will then be the average power of the pulse train at the output of the switching device. This technique eliminates the need for bulky and expensive power transformers and large heatsinks required by series pass transistors in an analog charging circuit.
A major problem with conventional switching-type charging circuits arises when circuits are used in high power applications. Switching type charging circuits typically have low-power factors. The power factor of a device describes the relationship of the relative phase of the input current and voltage when excited by an AC voltage and quantifies electrical losses which occur in a capacitive or inductive circuit. The power factor can be thought of as the ratio of the effective series resistance of a device to the complex impedance of the device and is expressed as a percent. A purely resistive device would have a unity power factor. Conventional switching-type charging circuits may have a power factor of 65% due to a widely varying input current demand and the constantly changing input voltage of an AC signal.
The relatively low-power factor of a switching-type battery charger becomes a problem when large amounts of power are required by a load.
As an example, suppose 1,000 watts DC were required from a supply to be operated from a 115 V AC, 15 A service. A typical switching-type charging circuit would run with a conversion efficiency of approximately 85 percent. Therefore, the power demand of this device would be 1,176 W. With a power factor of 65% the volt-ampere input to the device would be 1,809 VA. This translates to an input current of 15.73 amperes or 0.73 amps above the capacity of the supply service. If the power factor of switching device was near unity, the device would draw approximately 10.5 amperes and, therefore, be operable from the intended service.
For the foregoing and other shortcomings and problems, there has been a long-felt need to optimize the power factor of a switching-type charging circuit while maintaining the high efficiency, cool operation and relatively small size of the device.